This invention relates to methods and means for communicating between an upstream point and a number of downstream points, and particularly to communication systems starting at a source with optical fiber trunks and terminating at subscribers via coaxial cable distribution networks.
Fiber deployment in the trunk portion of distribution networks has resulted in a dramatic increase in options available to subscribers. Fiber feeders now end in nodes from which short coaxial cable distribution networks serve relatively small numbers of subscribers such as 100 to 2,000. Because the length of the coaxial cable in each case is less than approximately 1 to 2 kilometers, the effective available bandwidth is large, for example 1 GHz. This enhanced bandwidth allows a variety of services in addition to the conventional load of amplitude-modulation, vestigial-sideband, (AM-VSB) video channels. At present, fiber-coaxial systems broadcast substantially the same signals to each subscriber in the distribution plant. While this is fine for ordinary cable services, it fails to provide the privacy and security demanded for business, personal file transfers, pay video on demand, or special telephony. Privacy represents the inability of unintended listeners to eavesdrop. Security means that no rogue radio-frequency RF upstream transmitter can interrupt communication.
Copending U.S. application Ser. No. 08/029,724, filed Mar. 11, 1993 discloses a passive optical network (PON) subscriber loop system that avoids this limitation by taking advantage of tunable lasers and wavelength selective routers. This arrangement furnishes a virtually private channel between the headend station and each user. However, it requires distribution with fiber optic lines directly to each subscriber. U.S. Pat. No. 5,136,671 discloses suitable optical router for this purpose.
An object of the invention is to furnish a system allowing a coaxial cable distribution network to limit information broadcast to each subscriber, whether or not such information is encrypted and whether or not such information is from an optical fiber trunk.
Another object of the invention is to furnish the equivalent of a private downstream and upstream channel, between a broadcast source and a subscriber, which is inaccessible to other users.
Yet another object of the invention is to furnish RF routers suitable for connection to fiber optic devices and for frequency division multiplexing and demultiplexing RF carriers.